PVCz Films Research Articles

Efficient Perovskite Light‐Emitting Diodes with a Siloxane‐Blended Organic Hole Transport Layer

Metal halide perovskite light‐emitting diodes (LEDs) are promising for future display applications because of their excellent advantages, such as high external quantum efficiency, emission color tunability, and high emission color purity. Although solution processing widely used for perovskite film fabrication is an additional advantage, there are cases in which fabricating a perovskite‐emitting layer with spin‐coating results in the dissolution of an underlying organic hole transport layer (HTL). As a result, since a perovskite layer comes into partial contact with an indium tin oxide (ITO) anode layer, excited states formed in a perovskite‐emitting layer are quenched by charge transfer to ITO. In this study, it is shown that adding tetraethyl orthosilicate (TEOS) into a HTL material of poly(N‐vinylcarbazole) (PVCz) is an effective method used to overcome this issue. Upon heating, the hydrolysis reaction of TEOS molecules takes place in PVCz films to form a siloxane network, which makes PVCz films insoluble and, therefore, alleviates the excited‐state quenching. It is demonstrated that using the siloxane‐blended PVCz HTL increases external quantum efficiencies of perovskite LEDs to 15.4 ± 0.3% from the original 10.4 ± 0.3% by about 1.5 times.

Ellipsometric determination of optical properties of carbazole-containing poly(l-glutamate) thin films prepared from different solvents

In this study, the refractive indices (n) and thicknesses of carbazole-containing hole-transport materials such as poly(γ-carbazolylethyl l-glutamate) (PCELG) and poly(N-vinyl carbazole) (PVCz) films were determined by carrying out ellipsometric measurements. The thicknesses of PCELG and PVCz films determined by ellipsometric analysis were in good agreement with those determined by surface profilometry. The dependence of the refractive indices of the PCELG films on film thickness was classified into two types on the basis of the solvent from which the films were prepared: the refractive indices either increased with increasing film thickness, as in the case of PCELG films prepared from 1,2-dichloroethane (DCE) and monochlorobenzene (ϕ-Cl), or were independent of the film thickness, as in the case of films prepared from 1,1,2,2-tetrachloroethane (TCE). A comparison of these results with the structures of the polymers, as determined by 1H NMR, reveals that the two types of dependences of the refractive indices of the PCELG thin film on the film thickness can be attributed to the two types of aggregation structures and/or orientational characteristics corresponding to the helical conformation of the polymer. In contrast, the refractive indices of PVCz films are governed mainly by the film thickness. Finally, we would like to emphasize that the combination of ellipsometry and other techniques such as NMR and surface profilometry provide information not only on the film thickness and refractive index but also on the aggregation structure in thin films with thicknesses on the order of 50nm.

Giant magnetoresistance due to electron-hole pair mechanism in poly( N-vinylcarbazole)

An amorphous molecular semiconductor, poly( N-vinylcarbazole) (PVCz), exhibits negative giant magnetoresistance (MR) under ambient conditions. The application of a weak magnetic field of 10 mT to PVCz films doped with lumichrome, in which light excitation immediately creates triplet electron-hole (e-h) pairs that are precursors of photocarriers, causes a steep decrease in resistivity by more than 20% at ambient temperature. Further, the resistivity of the doped film gradually reduces by approximately one-half under a field of 1 T, equivalent to an MR ratio of −55%. In addition, anomalous spikes are also detected at 0.07, 0.30, and 9.0 T, indicative of an exponential dependence with a decay distance of 0.1 nm in the exchange interaction of the e-h pair. A quantum mechanical calculation based on the density operator formalism clarifies that the observed MR effect can be comprehensively understood by the spin-selective charge dynamics and the coherent and incoherent spin dynamics of the e-h pair in a quasi-one-dimensional lattice for photocarrier generation. Model calculations also indicate the importance of the spin-lattice relaxation for the giant MR effect in organic molecular semiconductors.

Photoinduced Electron Transfer Dynamics in Aromatic Vinyl Polymers and Related Systems: Time‐Resolved Detection of Primary Events

AbstractFor Abstract see ChemInform Abstract in Full Text.

Thin films formation of poly(N-vinylcarbazole) by laser ablation deposition

The laser-ablation deposition technique was examined for poly(N-vinylcarbazole), PVCz, which is a representative photoconductive aromatic vinylpolymer. Thin films were deposited under various irradiation conditions (laser wavelengths were 351, 308, and 248 nm and their fluences were 70, 300, and 450 mJ/cm2), and their chemical structures and surface morphology were revealed by means of infrared-absorption, fluorescence spectroscopy, and atomic force microscopy, respectively. They were quite sensitive to the laser fluence, as well as wavelength. A thin film of PVCz was formed only under an optimum irradiation condition where the laser fluence and wavelength were 300 mJ/cm2 and 351 nm, respectively. When the irradiation condition deviated from the optimum one, thin films contained carbazolyl derivatives such as N-alkylcarbazole and N-vinylcarbazole oligomers. It was revealed that the thin PVCz film was formed by a repolymerization due to radical mechanism. In addition, it was suggested that cross-linking reactions occurred in the films deposited at 248 and 308 nm. All the films were amorphous, and particle-like aggregates, with sizes varying with the irradiation conditions, were observed on their surface. It was demonstrated that the ablation-deposition technique is applicable to the formation of thin PVCz films.

Vapor-deposited poly(N-vinylcarbazole) films for hole transport layer in organic electroluminescent devices

Vapor-deposited films of poly(N-vinylcarbazole) (PVCz) were used as hole transport layers in organic electroluminescent devices. Hole transporting ability of the vapor-deposited PVCz films in the double-layered cell remarkably depended on their film morphology. Light emission was obtained only when a thin, homogeneous PVCz film (10–15 nm in thickness) was deposited on an anode substrate which was kept at 100°C during deposition. When the PVCz film was overcoated on a low molecular hole transporting layer, the EL performance of the mutilayered device was retained for a longer time as compared with that without the PVCz deposition. This improved durability was ascribed to the PVCz layer which prevents the low molecular layer from being degraded by aggregation.

Magnetic Field Effect on Laser Ablation of Organic Polymer Films as Revealed by Atomic Force Microscopy

A magnetic field effects on 308 nm excimer laser ablation of poly(N-vinylcarbazole) (PVCz) films was investigated by means of atomic force microscopy. The morphology of ablated surface exhibited characteristic modification by applying a magnetic field. An orientation pattern was observed on the polymer surface ablated in a horizontal magnetic field. PVCz films prepared by the cast-method in a magnetic filed were not oriented. A novel technique combining laser light and a magnetic field was developed for sub-micron surface patterning.

A Structural Study for Highly Efficient Electroluminescence Cells Using Perylene‐Doped Organic Materials

Highly efficient electroluminescence (EL) cells using perylene‐doped organic materials have been studied. Three types of cell structures were used for this study, and the device properties were compared with each other. As the light emitting layer, 60 to 70 nm thick perylene‐doped poly(N‐vinylcarbazole) [PVCz] was used. 2‐(4′‐tert‐butylphenyl)‐5‐(4′‐biphenyl)‐1,3,4,‐oxadiazole [Bu‐PBD], tris(8‐hydroxyquinoline)aluminum [Alq3] were used as a hole blocking layer and an electron‐transport layer, respectively. Copper phthalocyanine [CuPc] was used as a hole injection layer at the bottom of emitting layer. The wavelength at electroluminescence from the perylene‐doped PVCz films shifted to a long wavelength as the perylene concentration was increased. The electroluminescence due to perylene was observed at the wavelengths of 455, 480, and 520 nm. Blue and yellow green luminescences were observed for the perylene concentrations of 0.76 and 12.7 mole percent (m/o), respectively. From the band diagram study, it has been revealed that the barrier height for electron injection from MgAg electrode is 1.9 eV, and the barrier height for hole injection from indium‐tin oxide film is 0.9 eV. Although the activation energy obtained from current‐temperature characteristics was 0.36 eV for the low dopant concentration of 0.76 m/o, the activation energy for 12.7 m/o doped PVCz was 0.13 eV. This low activation energy for 12.7 m/o doped PVCz films is due to the carrier trap generation by the perylene molecules in the PVCz. The luminances and efficiencies for the EL cells with the carrier transport layers were over one order of magnitude improved. The maximum luminance for the EL cell with both electron and hole transport layers was 1900 cd/m2 at the applied voltage of 15 V, and the efficiency was 0.77 lm/W at the current density of 10 mA/cm2.

Electrochemical quartz crystal microbalance studies of the electrodeposition and subsequent cross-linking of poly(N-vinylcarbazole) films

Abstract Crystal impedance and electrochemical measurements were used to demonstrate that compact and rigid poly(N-vinylcarbazole) (PVCz) films may be obtained from anhydrous acetonitrile solutions, provided that the polymerisation potential is relatively low and the bath solution is enriched in H + ions. Electrochemical quartz crystal microbalance studies of PVCz films led to the conclusion that the break-in process includes cross-linking between the polymer chains via the vinyl groups with irreversible incorporation of anions. The apparent diffusion coefficients obtained for transport of ions through the film during reversible oxidation and irreversible cross linking of the polymer are, respectively, 6 × 10 −10 cm 2 s −1 and 2 × 10 −10 cm 2 s −1 .

Photoinduced Electron Transfer Processes of C60-Doped Poly(N-vinylcarbazole) Films As Revealed by Picosecond Laser Photolysis

Photoinduced electron transfer dynamics of poly(N-vinylcarbazole) (PVCz) films doped with a small amount of C60 was investigated by using picosecond and nanosecond laser flash photolysis techniques under various experimental conditions. From the time evolution of transient absorption spectra in the 600−1180 nm wavelength region, it was revealed for 0.7 mol % C60-doped PVCz films that the charge-separated state (PVCz+−C60-) between PVCz and C60 was produced immediately after excitation with a picosecond 532 nm laser pulse. The decay of the initial charge-separated state with a time constant of 1.2 ns comprised the following three channels: the charge-recombination process, the hole-migration reaction to the neighboring carbazolyl chromophores, and the formation of the local triplet state of C60 (3C60*). The reaction yield of the hole migration was estimated to be 30%. The present results were quite different from those reported previously for densely C60-doped PVCz films excited with a picosecond 355 nm l...

Picosecond−Microsecond Dynamics of Photoinduced Electron-Transfer Processes in Amorphous Solid Films of Dimeric Carbazolyl Compounds Doped with 1,2,4,5-Tetracyanobenzene

Picosecond and nanosecond transient absorption spectroscopy and picosecond transient dichroism measurements were applied for elucidating photoinduced charge-separation, charge-recombination (CR), and hole-transfer (HT) processes in amorphous solid films of carbazolyl dimeric compounds (1,3-di(N-carbazolyl)propane and 1,2-trans-di(N-carbazolyl)cyclobutane) doped with 1,2,4,5-tetracyanobenzene (TCNB). Time-resolved fluorescence spectra were also measured for elucidating fluorescent states responsible for recombination emission. These results were compared with those for poly(N-vinylcarbazole) (PVCz), where the concentration of carbazolyl (Cz) chromophores is the same as the above dimeric compounds. The ground-state charge-transfer (CT) absorption and the steady-state CT fluorescence spectra were different between the present dimeric and the PVCz films. In both the films, the CR process was in competition with the HT process in the initial charge-separated state produced via the photoexcitation of the ground...

Electrodeposition and electrochemical properties of poly(N-vinylcarbazole) films on platinum electrodes

Abstract The influence of the electrodeposition regime on the morphology of poly(N-vinylcarbazole) films and on the electrochemical properties of Pt/PVCz electrodes was studied. The polymer films were obtained potentiostatically on stationary and rotating disc Pt electrodes. It was found that the use of rotating electrodes allowed kinetic control of the growth process, and the resulting films were compact and adhered well to the substrate. The rate constant of the polymerization reaction and the diffusion coefficient of N-vinylcarbazole were calculated. The switching reaction of the PVCz film is discussed and related to the morphology of the film and to the composition of the solution. The permeability of the PVCz films and the electric conductivity of the polymer were estimated using the ferrocene/ferricinium redox couple.

Fluorescence dynamics of charge-transfer-complex films of poly( N-vinylcarbazole) and 1,2,4,5-tetracyanobenzene and molecular aspects of the disordered structure

Charge-transfer (CT) fluorescence of poly( N-vinylcarbazole) (PVCz) films doped with 1,2,4,5-tetracyanobenzene was investigated in comparison with exciplex (and/or exterplex) fluorescence of PVCz films doped with 1,4-dicyanobenzene by using the nanosecond time-correlated single-photon-counting method. For excitation of PVCz absorption band and CT absorption band, the fluorescence peak in the time-resolved spectra shifted continuously to short wavelength with time. The direction of the shift was opposite to that of the exterplex fluorescence. The dependence of the steady-state CT fluorescence intensity on concentration of the acceptor was also different from the exciplex system. These different behaviours of fluorescence dynamics between the CT and the exciplex systems were discussed from the viewpoint of the overlap of CT and exterplex fluorescence for the CT system and of a morphological change of PVCz films caused by formation of ground-state CT complexes.

The Pulse-Photoconductivity of Poly(1-vinylpyrene)

Abstract The migration of electronic excitation energy, the steady-state photoconductivity, the hole-drift mobility and the carrier-photogeneration efficiency of holes in poly(1-vinylpyrene)(PVPy) films were investigated and compared with those in poly(N-vinylcarbazole)(PVCz) films. Although the concentration of the excimer-forming sites in PVPy films was about twenty times as large as that in PVCz films, the magnitude of the steady-state photoconductivity of PVPy was comparable to that of PVCz. The hole mobility in PVPy films was about one-half as large as that in PVCz films. The electric field and temperature dependence of the mobility fit Gill’s empirical equation. The carrier-photogeneration efficiency, Φ, of PVPy was larger than that of PVCz. The filed dependence of Φ of PVPy for all the excitation bands was in good agreement with the Onsager theory of geminate recombination. The high photoconductivity of PVPy could be explained on the basis of these results.

Oxidative electropolymeriza tion of N-vinylcarbazole : Growth and electrochemical behavior of poly- N-vinylcarbazole films on Pt

The growth of poly- N-vinylcarbazole films obtained by electropolymerizing N-vinylcarbazole (VCz) onto a Pt electrode in acetonitrile with different supporting salts has been followed by cyclic scans with linear variation of potential. Film growth is dependent on the nature of the anion of the supporting electrolyte which associates with the oxidized form of the polymer; the smaller the anion, the faster the growth. Electropolymerizing VCz at a metal electrode yields films which start growing with the formation of dimer cation radicals and which stop growing with their electrochemical reduction, whereas polymerization of VCz in solution yields a precipitate the formation of which starts with protons released during crosslinking between carbazyl moieties. The doped form of PVCz films is a relatively good conductor, whereas the reduced form seems highly resistant and prevents the penetration of electroactive species (even in the case of very thin films).

Extrinsic Carrier-photogeneration in Poly(N-vinylcarbazole)

Abstract The carrier-photogeneration efficiency of holes in poly(N-vinylcarbazole) (PVCz) films was investigated by means of the time-of-flight method as a function of applied electric field, temperature, and excitation band. The extrinsic effects on the efficiency were examined; these included photooxidation of film surface and doping with a weak electron-acceptor, dimethyl terephthalate (DMTP). The experimental results were analyzed according to the Onsager theory of geminate recombination of a bound electron-hole pair with an interionic separation of ro. The primary yield of the electron-hole pair, φo, increased from 0.02 to 0.059 with decreasing excitation wavelength. By photooxidation, it increased up to 0.09 for all the excitation bands. The ro was independent of the excitation band, and was identical between fresh and photooxidized films. In DMTP(1.8 mol%)-doped films, both φo and ro were independent of the excitation band, although their values were different from those of undoped films. These results indicate that the extrinsic carrier-photogeneration via nonrelaxed exciplexes formed between singlet excitons and acceptor-like impurities or photooxidation products is predominant in PVCz films for all the excitation bands.

Photo-oxidation Effect of Poly(N-vinylcarbazole) Film on Photoconductivity and Electronic Spectra

Abstract The photo-oxidation effect on poly(N-vinylcarbazole) (PVCz) and amorphous 1,3-di(N-carbazolyl)propane films was studied by means of ultraviolet absorption, infrared absorption, and fluorescence spectra. The photoconductivity of the photo-oxidized PVCz films was also measured as a function of photoirradiation time. A photo-oxidation product which formed near the surface of films had a carbonyl substituent group, acted as a singlet exciton trap in competition with the excimer-forming sites, and formed an exciplex with an excited carbazolyl chromophore. The very slight amount of the photo-oxidation product enhanced the photoconductivity of PVCz films by a factor of ca.15, suggesting that the product acted as an electron-accepting impurity in the photo-carriergeneration mechanism in the lowest π-π* absorption region of PVCz films.

Singlet Excitation Energy Transfer in the Vinyl Polymers with Pendant Carbazolyl Groups

Abstract The migration of electronic excitation energy in films of poly(N-vinylcarbazole) (PVCz) prepared by the radical and cationic polymerizations(PVCz(r) and PVCz(c) respectively), poly[2-(9-carbazolyl)ethyl vinyl ether] (PCz-EVE), brominated PVCz(BPVCz), and poly(9-acryloylcarbazole) (PACz) has been studied by means of fluorescence-quenching experiments, using dimethyl terephthalate or perylene as a guest molecule. No clear difference in the concentration of the effective intrinsic trap sites was observed between PVCz(r) and PVCz(c) films. The concentration of the sandwich-like excimer site(cE) in a PVCz(r) film was nearly equal to that in a PVCz(c) film, while the concentration of the second excimer site (cS) in a PVCz(r) film was higher than that in a PVCz(c) film by a factor of about 1.6. The value of cE was much larger than that of cS for both films. The second excimer site seems to be a shallow trap. The number of carbazolyl chromophores covered by a singlet exciton during the lifetime was in the following order: PCzEVE>PVCz(r)≈PVCz(c)>PACz≈BPVCz. This order was explained by the concentration of the intrinsic trap sites depending on the distance between neighboring Cz chromophores, the lifetime of the singlet exciton, and the concentration of extrinsic trap sites.

The Photoconductivity of Poly(N-vinylcarbazole). II. Dark Conductivity in a Sandwich-type Cell

Abstract The dark conductivity in a sandwich-type cell of PVCz films was investigated in a high vacuum, taking the temperature, the applied voltage, and the film thickness as parameters. In low fields, the conductivity was proportional to the applied voltage. In high fields, however, it was proportional to the fourth power of the applied voltage. This nonohmic dark conductivity in high fields may be attributed to the Pool-Frenkel effect. A model of the energy-level diagram is presented which can explain the dark conductivity in both low and high fields.